1. Field of the Invention
The present invention relates to a driving method for reducing image sticking effect of display images, and more specifically, to a driving method for reducing image sticking effect of images on a liquid crystal display (LCD).
2. Description of the Prior Art
FIG. 1 is a diagram illustrating a cross-sectional view of a conventional liquid crystal display (LCD) 100. As shown in FIG. 1, the LCD 100 comprises two glass substrates, G1 and G2, and a liquid crystal (LC) layer L1 disposed between the glass substrates G1 and G2. A plurality of data lines (not shown) and a plurality of scan lines (not shown) are laid on the glass substrate G1 and are interwoven each other to form a plurality of the pixel areas. The liquid crystal layer L1 comprises liquid crystal molecules X, of which the rotation can be controlled by applying voltage. In ideal condition, the LC layer L1 only contains liquid crystal molecules X only. However, some other particles, namely impurities P, also exist in the liquid crystal layer L1. The impurities P, as shown in FIG. 1, can be ions with positive or negative charges, or neutral molecules with certain polarities.
FIG. 2 is a diagram illustrating the general driving method of the conventional LCD 100 to display an image. As mentioned above, the pixel areas are formed by interweaving data lined and scan lines and therefore, the pixel areas are indexed as Pmn where m and n indicate the number of the data line and scan line which are responsible for driving the pixel Pmn. The data voltages carried by the data lines correspond to the displayed image. However, only when the scan line Sn turns on, the data voltages on the data line Dm is input into the pixel area Pmn. For example, the data voltage on the fourth data line D4 will be input into pixel area P43 when the third scan line S3 turns on, and so forth. Therefore, the LC molecules in the pixel P43 will rotate according to the data voltages on the fourth data line D4 when the third scan line S3 turns on. Furthermore, when the scan line turns off, the data voltages on the data lines are not input into the pixels, and the liquid crystal molecules X in this pixel remain the state caused by the previous data voltages on the data lines. There are always data voltages on the data lines but the scan lines will sequentially turn on from G1 to Gn. As a result, an image is fully displayed on the screen while all data voltages on data lines are input into the pixels. The duration which this sequential process takes to display an image is called a “frame time”. Subsequently, the next frame starts while turning on the first scan line S1 to the last scan line Sn to show the next image, and so forth. In general, between two frames, there is a moment when all of the scan line turns off, which is so-called “blanking time”.
FIG. 3 is a diagram illustrating the relation between the rotation of the liquid crystal molecules X and the data voltages Vd on the data lines in more detail. In reality, one end of the pixel areas is connected to the data line where a data voltage Vd is applied, and the other end of the pixel is connected to the other glass substrate G2 where a fixed common voltage Vcom is applied. Therefore, the actual voltage sensed by the liquid crystal molecules X in the pixel is the relative voltage difference between the data voltage Vd and the common voltage Vcom. This relative voltage difference is the real factor that determines the rotation of the liquid crystal molecules X.
FIG. 4 is a diagram illustrating the distribution of the impurities P after the conventional LCD 100 displays an image for a period of time. If the data voltages Vd on the data lines were perfectly symmetric AC (alternative current) waveform relative to the common voltage Vcom, the net movement of the impurities P would be zero and their distribution would remain as the initial condition. Nevertheless, the data voltages are slightly asymmetric AC waveforms unavoidably so that a net DC voltage is formed after displaying an image for a period of time. This DC voltage induces the positive-polarized impurities P moving and gradually accumulating at one side of the LC layer L1 while the negative-polarized impurities P accumulate at the other side of the LC layer L1. These accumulated impurities P generate an inner electric field E in the liquid crystal layer L1, which shields off the following data voltage to apply on the liquid crystal molecules X. Consequently, the liquid crystal molecules X cannot rotate to the correct direction and the image sticking problem occurs.
FIG. 5 is a diagram illustrating the distribution of impurities P after the conventional LCD 100 displays images for a period of time. Besides the net DC voltage, the movement of the impurities P are affected by the directions of the liquid crystal molecules X as well. As shown in FIG. 5, the liquid crystal molecules X points at a specific direction which is determined by the voltage difference V between data voltage Vd and common voltage Vcom. Such a direction causes the horizontal movements of the impurities P other than the vertical movements. The impurities P therefore accumulate to form a “boundary” in the LC layer L1 if the movements described above remain for a period of time. The impurities-formed boundaries in the LC layer L1 distort the input voltage so that an abnormal image appears near the boundary which is the so-called line-shape image sticking.